Charger STV Performance

Hello,

I own and operate a Canadian Built 1988 (20' 6") Charger STV Modified Tunnel Hull boat. It is equipped with a CMC Jack Plate. I have recently purchsed a new 2002 Mercury 2.5 L Sport High performance racing application engine. Max RPM 7800. The 2.5 replaced a 200 HP 1988 Mercury engine, equipped with a 28 Pitch Chopper. Mercury suggested a 26 pitch Chopper on the new engine, the problem is that to this point, the old engine has outperformed the new high performance engine.

Old  5400 rpm, 78mph. New 73 mph, max achievable rpm 6800. Max speed generated at 6200 rpm. the engine/prop combination appears to provide ample torque and bow uplift, but disappointing high performance. Any advice would be greatly appreciated.

Thank you
LAS

 

LAS,

Don't feel bad as the same thing has happened to me many times. The 28 prop is always going to be more efficient than the 26 or the 24 you need. It takes a lot of horsepower to overcome the lost efficiency. So there is some bad news. I suspect that the new motor has a sportmaster and the old one had a nosecone gearcase. There is some more MPH. It sounds like your boat is hyper sensitive to little changes. Here's the easiest fix. Get a 2 to1 ratio nosecone 200 gearcase and run your old prop. The rpm would be around 7000 and probably faster than the old 200. You could try 2 to 1 gears in the sportmaster but I know it will not match the 200 coned case. You could find some miracle 25 or 26 prop but I think it will be a hard task. You could try some different props and heights and let me know the feedback. I think your old combo was very good and to go back to the gearcase and prop would be the most accurate. We need the gear change to get the rpm up.

Try some things and let me know. If little improvements start happening I can help you go find the rest of the speed.
Randy

 

 

Please submit whatever questions you have to IHRTechTalk@yahoo.com.  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Thanks,
Mark C.

 

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